The well-known lithographic printing plates depend for their success on the fact that image areas attrack ink and repel water (thus oleophilic) while nonimage areas repel ink and attract water (thus hydrophilic). This is true regardless of whether the plate is negative acting, i.e., the light-sensitive coating is hardened upon exposure to actinic light, so that the unaffected portions are removed by developer, or positive-acting, so that the light-sensitive coating is rendered soluble by exposure to actinic light and may be removed by developer.
It is desirable that the image area be highly oleophilic, so that after mounting a plate upon a press, the image areas accept ink from the ink rollers and transfer ink to the blanket and thence to paper in the fewest number of impressions. This capability is known as fast roll-up, and is an economic factor because of the loss due to wasted paper and machine time if the image only slowly accepts and transfers ink. This condition is known as blinding.
The nonimage area, as noted above, must be hydrophilic initially before the plate is mounted on the press and throughout the press run. An incompletely hydrophilic nonimage area will accept some ink and produce a greyed or ink-smudged background, a condition known as scumming.
To minimize either blinding or scumming, it is conventional to treat a freshly developed plate after water rinsing with a plate finisher which normally contains a hydrophilic colloid, a surfactant, salts and water. This action is called desensitization.
Gum arabic and synthetic gums have been used to finish lithographic printing plates. However, with these agents, gum blinding often occurs.
Improved finishers have been prepared and used which contain water, tapioca dextrin, an anionic surfactant, and as a humectant, glycerol. Sporadic blinding occurred despite the improvement; performance was not consistent. The tapioca dextrin used required prolonged heating to dissolve it.
Another improved finisher is described in U.S. Pat. No. 4,162,920, which finisher is additionally a preserver. This finisher uses tapioca dextrin as the hydrophilic colloid. Other ingredients are a mixture of anionic and nonionic surfactants, glycerol and a petroleum distillate to dissolve the nonaqueous surfactant. An emulsion is formed.
Despite freedom from blinding, as it is an emulsion, this finisher settles on standing and is not usable for machine processing.
U.S. Pat. No. 4,143,021 describes an aqueous finisher which uses a polyoxyalkylene glycol to eliminate tackiness of drying films on lithographic printing plates encountered in patentee's previous formulation. However, applicants have never encountered a tackiness problem in previous finishers.
It is an object of this invention to provide an aqueous composition that will desensitize the nonimage areas of a lithographic printing plate while simultaneously enhancing the receptivity to ink of the image areas with a minimum number of impressions on the press. It is another object of this invention to provide a composition of simplified manufacture, handling and storage. It is a further object to provide a composition which can be used in a processing machine as well as by hand, which can be readily removed with a water rinse or by contact with the dampening rollers of a lithographic press. It is also an object to provide all of the previously mentioned characteristics in a composition which is dependable and reliable in its performance.